The present invention relates to a wireless communication system, wireless communication apparatus, wireless communication method, and computer program for establishing communication among a plurality of wireless stations that constitute, for instance, a wireless LAN (Local Area Network), and more particularly to a wireless communication system, wireless communication apparatus, wireless communication method, and computer program for allowing various communication stations to perform network operations in an autonomous distributed manner without using any specially installed control station.
More specifically, the present invention relates to a wireless communication system, wireless communication apparatus, wireless communication method, and computer program for sharing the same frequency band with a radar wave system by making a frequency change (DFS: Dynamic Frequency Selection) in response to a radar wave detection in an autonomous distributed communication environment, and more particularly to a wireless communication system, wireless communication apparatus, wireless communication method, and computer program for efficiently detecting a radar wave and making a frequency change (dynamic frequency selection) in an autonomous distributed network while considering, for instance, the power consumption of each communication station.
In recent years, the demand for a wireless LAN system has remarkably increased because its increased speed and reduced price. Especially, the introduction of a personal area network (PAN) is recently studied in order to provide information communication by establishing a small-scale wireless network among a plurality of electronic devices existing around a person. For example, different wireless communication systems are defined by using a 2.4 GHz, 5 GHz, or other frequency band that does not require licensing from a supervisory authority.
Typical standards concerning wireless networks are IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (refer, for instance, to Non-patent Document 1), HiperLAN/2 (refer, for instance, to Non-patent Document 2 or 3), and IEEE 302.15.3, Bluetooth. As regards the IEEE 802.11 standard, there are extended standards such as IEEE 802.11a (refer, for instance, to Non-patent Document 4), IEEE 802.11b, and IEEE 802.11g, which apply depending on the employed wireless communication method and frequency band.
To configure a local area network by using a wireless technology, one control station apparatus called an “access point” or “coordinator” is generally installed within an area so that a network is formed under the overall control of the control station.
When a certain information apparatus transmits information to a remote apparatus within a wireless network in which an access point is installed, a band reservation based access control method is widely used. When such a method is used, a band required for information transmission is reserved at the access point so as to use a transmission path while avoiding collision with an information transmission by another communication apparatus. In other words, the access point is installed so as to provide synchronous wireless communication in which communication apparatuses within the wireless network synchronize with each other.
However, when asynchronous communication is to be established between a transmitting communication apparatus and a receiving communication apparatus in a wireless communication system in which an access point exists, wireless communication via the access point is essential. It means that the transmission path use efficiency is reduced to half.
Meanwhile, an “ad-hoc communication” method is devised as a wireless network configuration method. When this method is used, terminal devices establish wireless communication directly and asynchronously. Particularly in a small-scale wireless network comprising a relatively small number of clients that are located in the vicinity of each other, the use of the ad-hoc communication method seems appropriate because wireless communication can be established directly and asynchronously by any terminal devices without using a specific access point.
In an IEEE 802.11 wireless LAN system, for instance, IEEE 802.11 networking is based on the concept of BSS (Basic Service Set). There are two types of BSSs. One BSS is defined by an “infra mode” in which an AP (Access Point: control station) or other master exists. The other one is IBSS (Independent BSS), which is defined by an “ad-hoc mode” in which only a plurality of MTs (Mobile Terminals: mobile stations) exist. In the ad-hoc mode, a peer-to-peer operation is performed in an autonomous distributed manner even when no control station is provided. When a beacon transmission time has come, each terminal device performs random period counting. The local terminal device transmits a beacon if it does not receive the beacon of a remote terminal device before the counting ends.
In an ad-hoc wireless communication system, no central control station exists. Therefore, the ad-hoc wireless communication system is suitable for configuring a home network that comprises, for instance, electrical home appliances. The ad-hoc network does not readily fail because it automatically changes the routing even when one electrical home appliance becomes faulty or turns off. Further, the ad-hoc network can transmit data to a relatively remote location without lowering a high data rate by hopping packets multiple times between mobile stations. There are various known examples of a developed ad-hoc system (refer, for instance, to Non-patent Document 5).
The IEEE 802.11a system operates in a 5 GHz band (5.15 to 5.35 GHz and 5.47 to 5.825 GHz). It uses the same frequency band as for meteorological radar waves. Therefore, recommendation ITU-R SA. 1632 is made by the WRC-03 conference, which determines international radio frequency allocations. The recommendation stipulates that a frequency band equivalent to a radar wave be shared by requiring the wireless LAN side to avoid radar wave interference. More specifically, it is required that a wireless LAN access point using a 5.25-5.35 GHz/5.47-5.725 GHz band have a radar wave detection function, notify all controlled mobile stations, upon radar wave detection, of a change in the employed frequency, and cause a change in the employed frequency. This frequency change operation is referred to as a DFS (Dynamic Frequency Selection).
A sequence of radar wave detection and DFS related operations will now be described with reference to FIGS. 23 through 25.
FIG. 23 shows a typical configuration of a wireless LAN network. In the example shown in FIG. 23, the network comprises one access point 1110 and four mobile stations 1200. It is assumed that the mobile stations 1200a-1200d communicate with the access point 1100 and are controlled by the access point 1100. All communications are provided via the access point. Even when mobile stations 1200a and 1200b establish data communication, it is established via the access point.
FIG. 24 schematically shows a typical functional configuration of the access point 1100. To achieve radar wave detection, the access point 1100 incorporates a radar wave detection section 1130. The radar wave detection section 1130 keeps on checking for an incoming radar wave at almost all times (except for a period during which the local station is engaged in transmission). If a predetermined threshold value is exceeded by detected radar wave strength, the radar wave detection section 1130 notifies a control section 1140 of such an event. Upon receipt of such a notification, the control section 1140 creates command data for causing its controlled mobile stations 1200 to apply a frequency change, modulates the command data, and transmits the resulting radio signal to a wireless LAN transmission/reception section 1120 via an antenna 1110. The changed frequency channel number data is written within the command data.
FIG. 25 schematically shows a typical functional configuration of a mobile station 1200. The mobile station 1200 shown in the figure receives the transmitted radio signal with an antenna 1210, demodulates the radio signal with a wireless LAN transmission/reception section 1220, and conveys the received command data to a control section 1240. The control section 1240 creates response data for acknowledging the receipt of the command data, causes the wireless LAN transmission/reception section 1220 to modulate the command data, and transmits the resulting radio signal from the antenna 1210.
The access point 1100 uses the antenna 1110 to receive the radio signal from the mobile station 1200, causes the wireless LAN transmission/reception section 1120 to demodulate the radio signal, and conveys acknowledgment data from the mobile station 1200 to the control section 1140.
When the acknowledgment is received from each mobile station 1200, the access point 1100 changes the setting for the wireless LAN transmission/reception section 1120 to transmit/receive a frequency corresponding to the previously reported frequency channel number, and begins to perform a transmission/reception operation on a new frequency channel.
When the wireless LAN transmission/reception section 1220 detects that a periodic beacon from the access point 1100 cannot be received, the mobile station 1200 notifies the control section 1240 that such a periodic beacon cannot be received. Upon receipt of such a notification, the control section 1240 changes the setting for the wireless LAN transmission/reception section 1220 so as to transmit/receive a frequency corresponding to the previously reported, changed frequency channel data, and begins to perform a reception operation on a new frequency channel.
The above example of a radar wave detection/DFS function operation is a typical implementation in the infra mode in which a network operation is performed under the control of one access point. As is obvious from the comparison between FIGS. 24 and 25, the access point side performs the radar wave detection and DFS operations in an integrated fashion. The mobile station side is not equipped with a radar wave detection section.
Some technologies for avoiding radar wave interference in a network were already proposed (refer, for instance, to Patent Documents 1, 2, 3, and 4). All these technologies relate to a radar wave detection method for use in a network where an access point exists, that is, for use in the infra mode, or to a method for changing the frequency to avoid interference from a detected radar wave. In a system comprising an access point and a plurality of mobile stations, for instance, a “hidden base station” problem may arise so that a radar wave reaches the mobile stations but does not reach the access point. However, when a radar detection apparatus is installed in addition to the access point, it is possible to solve the “hidden base station” problem for radar waves (refer, for instance, to Patent Document 3).
Meanwhile, the aforementioned ad-hoc wireless communication system, in which each communication station operates in an autonomous distributed manner while no access point or base station is installed, has attracted a great deal of attention in recent years. However, no radar wave detection/interference avoidance technology has been developed for use in such an ad-hoc wireless communication system.
When, for instance, every communication station incorporates a radar wave detection section and constantly exercises its radar wave detection function, it is possible to avoid interference in an autonomous distributed manner. In this instance, however, it is necessary to supply electrical power to each communication station for radar wave detection purposes. Therefore, the overall system power consumption increases. At a battery-powered mobile station, for instance, the operating time decreases due to power consumption for radar wave detection so that the degree of user-friendliness decreases. To maintain an adequate length of operating time while the radar wave detection function is being exercised, there is no alternative but to increase the battery capacity. However, such an increase in the battery capacity goes against the trend toward reducing the device size, weight, and price.
[Patent Document 1] Japanese Patent No. 3461779
[Patent Document 2] Japanese Patent JP-A No. 300102/2002
[Patent Document 3] Japanese Patent JP-A No. 135831/2002
[Patent Document 4] Japanese Patent JP-A No. 285301/2001
[Non-patent Document 1] International Standard ISO/IEC 8802-11: 1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent Document 2] ETSI Standard ETSI TS101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-patent Document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
[Non-patent Document 4] Supplement to IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band
[Non-patent Document 5] C. K. Tho, “Ad Hoc Mobile Wireless Network” (Prentice Hall PTR)